1.26. Wave Amplitude www.ck12.org 1.26 Wave Amplitude Define wave amplitude. State how to measure the amplitude of transverse and longitudinal waves. Explain what determines the amplitude of a wave. On a windy day, moving air particles strike these flags and transfer their energy of motion to particles of fabric. The energy travels through the fabric in waves. You can see the waves rippling through the brightly colored cloth. The windier the day is, the more vigorously the flags wave. 90
www.ck12.org Chapter 1. Energy What s the Matter? Waves that travel through matter such as the fabric of a flag are called mechanical waves. The matter they travel through is called the medium. When the energy of a wave passes through the medium, particles of the medium move. The more energy the wave has, the farther the particles of the medium move. The distance the particles move is measured by the wave s amplitude. What Is Wave Amplitude? Wave amplitude is the maximum distance the particles of the medium move from their resting positions when a wave passes through. The resting position of a particle of the medium is where the particle would be in the absence of a wave. The Figure 1.59 show the amplitudes of two different types of waves: transverse and longitudinal waves. In a transverse wave, particles of the medium move up and down at right angles to the direction of the wave. Wave amplitude of a transverse wave is the difference in height between the crest and the resting position. The crest is the highest point particles of the medium reach. The higher the crests are, the greater the amplitude of the wave. In a longitudinal wave, particles of the medium move back and forth in the same direction as the wave. Wave amplitude of a longitudinal wave is the distance between particles of the medium where it is compressed by the wave. The closer together the particles are, the greater the amplitude of the wave. You can simulate waves with different amplitudes in the animation at this URL: http://sci-culture.com/advancedpoll/ GCSE/sine%20wave%20simulator.html Q: What do you think determines a wave s amplitude? A: Wave amplitude is determined by the energy of the disturbance that causes the wave. Energy and Amplitude A wave caused by a disturbance with more energy has greater amplitude. Imagine dropping a small pebble into a pond of still water. Tiny ripples will move out from the disturbance in concentric circles. The ripples are lowamplitude waves with very little energy. Now imagine throwing a big boulder into the pond. Very large waves will be generated by the disturbance. These waves are high-amplitude waves and have a great deal of energy. Summary Wave amplitude is the maximum distance the particles of the medium move from their resting positions when a wave passes through. Wave amplitude of a transverse wave is the difference in height between a crest and the resting position. Wave amplitude of a longitudinal wave is the distance between particles of the medium where it is compressed by the wave. Wave amplitude is determined by the energy of the disturbance that causes the wave. A wave caused by a disturbance with more energy has greater amplitude. Vocabulary wave amplitude: Maximum distance the particles of a medium move from their resting positions when a wave passes through. 91
1.26. Wave Amplitude www.ck12.org FIGURE 1.54 Practice Read about wave amplitude at the following URL. Examine the graphs and answer the two questions. Click on the links to see if your answers are correct. http://cse.ssl.berkeley.edu/light/measure_amp.html#measure4 Review 1. Define wave amplitude. 2. What is the amplitude of the transverse wave modeled in the Figure 1.55 if the height of a crest is 3 cm above the resting position of the medium? 3. Which of these two longitudinal waves has greater amplitude? (See Figure 1.56) 4. Relate wave amplitude to wave energy. 92
www.ck12.org Chapter 1. Energy FIGURE 1.55 FIGURE 1.56 93
1.27. Wavelength www.ck12.org 1.27 Wavelength Define wavelength. Describe the wavelength of transverse and longitudinal waves. Relate wavelength to the energy of a wave. Nobody really has such colorful eyes! The colors were added digitally after the photo was taken. They represent all the different colors of light. Light is a form of energy that travels in waves. Light of different colors has different wavelengths. Defining Wavelength Wavelength is one way of measuring the size of waves. It is the distance between two corresponding points on adjacent waves, and it is usually measured in meters. How it is measured is a little different for transverse and longitudinal waves. 94 In a transverse wave, particles of the medium vibrate up and down at right angles to the direction that the wave travels. The wavelength of a transverse wave can be measured as the distance between two adjacent crests, or high points, as shown in the diagram below.
www.ck12.org Chapter 1. Energy FIGURE 1.57 In a longitudinal wave, particles of matter vibrate back and forth in the same direction that the wave travels. The wavelength of a longitudinal wave can be measured as the distance between two adjacent compressions, as shown in the diagram below. Compressions are the places where particles of the medium crowd close together as the energy of the wave passes through. FIGURE 1.58 At the following URL, watch the animation to see examples of wavelength. Also, get a feel for wavelength by playing with the wave generator. http://earthguide.ucsd.edu/wav/wavelength.html Wavelength and Wave Energy The wavelength of a wave is related to the wave s energy. Short-wavelength waves have more energy than longwavelength waves of the same amplitude. (Amplitude is a measure of how far particles of the medium move up and down or back and forth when a wave passes through them.) You can see examples of transverse waves with shorter and longer wavelengths in the Figure 1.59. Q: Of all the colors of visible light, red light has the longest wavelength and violet light has the shortest wavelength. Which color of light has the greatest energy? A: Violet light has the greatest energy because it has the shortest wavelength. 95
1.27. Wavelength www.ck12.org FIGURE 1.59 Summary Wavelength is one way of measuring the size of waves. It is the distance between two corresponding points on adjacent waves, usually measured in meters. The wavelength of a transverse wave can be measured as the distance between two adjacent crests. The wavelength of a longitudinal wave can be measured as the distance between two adjacent compressions. Short-wavelength waves have more energy than long-wavelength waves of the same amplitude. Vocabulary wavelength: Distance between two corresponding points of adjacent waves, such as the distance between two adjacent crests of a transverse wave. Practice At the following URL, read about the anatomy of a wave. Then do the Check Your Understanding questions at the bottom of the Web page. Be sure to check your answers. http://www.physicsclassroom.com/class/waves/u10l2a.cfm Review 1. What is the wavelength of a wave? 2. Draw a simple transverse wave and label the wavelength. 96